1. Field of the Invention
This invention relates to semiconductor integrated circuit processes and the devices formed thereby. In particular, it relates to the capability of forming active devices e.g. transistors, in a suitable layer constituted of annealed polysilicon that has been formed on a silicon dioxide insulator.
2. Background Information
Although the development of integrated circuitry has proceeded chiefly on the basis of the availability of monocrystalline substrates, particularly of silicon, it has been recognized that a limitation exists with conventional techniques; namely, that only two-dimensional integrated circuits can be formed in such substrates or, as is mostly usually done, in epitaxial layers grown on such substrates.
Great interest has focused on polysilicon as providing an answer to the aforenoted limitation. Thus, because polysilicon supports further growth of silicon dioxide thereon, if the polysilicon can be suitably treated, such as by annealing to make the treated areas suitable for device construction, then entire circuits fabricated in such annealed polysilicon may be covered with oxide and successive layers of polysilicon can be built up to achieve further levels of circuitry.
The reason unannealed polysilicon is not suitable for device formation is that it is constituted of randomly oriented crystal grains on the order of 500 angstroms across and hence, carrier flow is impeded at each grain boundary with resulting low mobility which yields poor device characteristics.
In order that the present invention may be fully understood, reference may be made to certain background material, particularly to an article in Electronics for Nov. 22, 1979 in which at page 39 it is reported that devices have been formed which are said to have performance capabilities comparable to devices produced in silicon on sapphire (SOS), such devices being formed on easily grown silicon oxide. It is indicated in that article that isolation produced by the silicon oxide might be comparable with the excellent isolation already achieved by the use of sapphire substrates.
Whatever the merits and the possible advantages of the devices and of the technique described in the referenced articles, the building or fabrication of those reported devices begins with a single crystal P type silicon substrate. Accordingly, the starting point is with an expensive substrate.
It is therefore the primary object of the present invention to capitalize on the already known capability for forming active devices in polysilicon that has been formed on an insulator, but to do so utilizing a much less expensive substrate, namely silicon ribbons which are constituted of semicrystalline material grown according to already developed techniques. The techniques of their fabrication can be appreciated by reference to U.S. Pat. Nos. 4,075,055 and 4,116,641, assigned to the assignee of the present application.
Not only is a substantial cost reduction provided by the unique fabrication process of the present invention, involving the utilization of silicon ribbon as a substrate. Continuous processing capability is afforded because of the availability of extended ribbons. Moreover, such ribbons inherently have a unique defect structure which favors strain-induced recrystallization of the polysilicon layer deposited on the silicon oxide grown on the ribbon surfaces. As a result, larger grains are obtained than those realized prior to recrystallization. The same fundamental result is also achieved when single crystal substrates are employed--but of course at much greater cost and requiring techniques such as plasma etching for developing the needed uniform recrystallization.